Predistortion techniques have been used with both single carrier and multi-carrier radio frequency (RF) power amplifiers to correct for distortion resulting from amplitude-to-amplitude (AM-AM) and amplitude-to-phase (AM-PM) non-linearities in such amplifiers. Typically, non-linearities result in intermodulation distortion (IMD) products and/or adjacent channel power (ACP). Thus, ACP is one parameter that may be used to judge the effectiveness of a predistortion technique, the reduction in ACP being attributable to a predistorter.
However, predistorters are not useful in all communications systems. For example, RF power amplifiers used in Global System for Mobile Communication (GSM) and/or Enhanced Data Rates for Global Evolution (EDGE) systems have not benefited from predistorters primarily due to two reasons. First, stringent IMD requirements for GSM and EDGE systems often force the use of single carrier RF power amplifiers. Second, GSM and EDGE waveforms typically have low peak to average power ratios, benefiting more so from higher efficiency single carrier power amplifiers than other modulation schemes, such as Code Division Multiple Access (CDMA), Universal Mobile Telephone System (UMTS), etc. Thus, the low peak to average power ratio of EDGE and GSM waveforms, as well as other systems using phase modulation allow the use of RF power amplifiers operating close to the saturated power output rating of the amplifiers without significant degradation in ACP performance.
Another parameter that may be used to judge the effectiveness of a predistorter when used with a RF power amplifier is referred to as Error Vector Magnitude (EVM). EVM is a measurement of the amount of error remaining in a signal due to magnitude and phase distortion. In RF power amplifiers, the distortion effects due to changes in phase, i.e., AM-PM, begin at lower power levels than distortion effects due to changes in amplitude, i.e., AM-AM. Therefore, even though ACP requirements may be satisfied, in order to reduce the EVM, the RF power amplifier need be operated at reduced power levels. Such reduced power levels reduce the efficiency of the RF power amplifier, while simultaneously increasing system costs.
Since EDGE and/or GSM systems often use separate power amplifiers for each RF channel, there is a desire to keep cost of these amplifiers low, and their efficiency high. Thus, the cost and efficiency for these single carrier amplifiers has prevented the use of predistorters since it is often cheaper to add more output power capability to the single channel RF power amplifiers than to implement predistorters.
There is a need for a low cost adaptation of a polynomial predistorter for linearizing the phase response of a single channel RF power amplifier, and that allows operation of the amplifier at power levels close to saturation.